Hard PZT ferroelectric materials such as PZT4 and PZT8 have been the mainstay in last half century for high power applications. However, the lead content in PZT type ceramics is an environmental concern in electronic devices. For example, the European Union is proposing directives on waste from electrical and electronic equipment as well restrictions on hazardous substances and end-of life vehicles. The USA and Japan are expected to issue similar environmental regulations. It therefore is desirable to develop lead-free piezoelectric ceramics to replace lead-based materials.
Lead-free ceramic compounds may be categorized into three primary types, all of which have the ABO3 perovskite formulation: (1) BaTiO3 (“BT”), (2) K0.5Na0.5NbO3 (“KNN”) and (3) Na0.5Bi0.5TiO3 (“NBT”). These compounds, however, either display low TC (≦120° C.), show low piezoelectric activity, multiple polymorphic phase transitions as well as depolarization temperature which limit their utility. Various properties of these compounds are shown in Table I. In Table I, KCN is K4CuNb8O23 and MPB is Morphotropic Phase Boundary.
TABLE IDielectric and Piezoelectric Properties of Lead-free Piezoelectricsd33TcTO-T/Materialεr/ε0loss(pC/N)kok33(° C.)Td (° C.)QBaTiO317000.011900.360.51150100BaTiO3—CaTiO3—Co14200.0051500.310.46105−45800(K0.5Na0.5)NbO3 (HP)5000.021270.460.6420200240(K0.5Na0.5)NbO32900.04800.350.51420195100KNN-Li (7%)9500.0842400.450.64460~20/KNN-Li3%; Ta20% (LF3)920-12560.024-0.02190-2300.46-0.5050.62310-32350-70/KNN-LF4*1570/4100.61/25325/KNN-SrTiO3 (5%)950/2000.37/2772770KNN-LiTaO3 (5%)5700.042000.36/4305550KNN-LiNbO3 (6%)5000.042350.420.614607050KNN-LiSbO3 (5%)12880.0192830.50/3924540KNN-KCN2900.006900.360.554101901500NBT-KBT-LBT15500.0342160.401/350160/NBT-KBT-BT8200.031450.1620.519302224110NBT-KBT-BT (MPB)7300.021730.330.59290162150PZT5A17000.023700.600.71365/75PZT5H34000.026000.650.75193/75
Lead-free ceramic compounds such as solid solutions of NBT with K0.5Bi0.5TiO3 (“KBT”), NBT-KBT-BT, NBT-KBT- with Li0.5Bi0.5TiO3 (“LBT”) show a morphotropic phase boundary analogous to PZT and relaxor-PT systems. NBT-KBT, NBT-KBT-BT, and NBT-KBT-LBT, however, exhibit a nonpolar antiferroelectric phase transition temperature that occurs below their TC that limits their temperature range of use. Lead-free ceramic compounds such as KNN—LiNbO3 (“KNN-LN”), KNN—LiTaO3 (“KNN-LT”), KNN—LiSbO3 (“KNN-LS”), and KNN—Sr(Ba)TiO3 have piezoelectric properties comparable to hard PZT ceramic compounds. However, these KNN type lead-free compounds exhibit low mechanical quality factor Q and a shift in the orthorhombic-tetragonal polymorphic phase transition temperature from about 200° C. to about room temperature. This polymorphic phase transition significantly limits their utility due to property variations.
A need therefore exists for high performance lead free piezoelectric ceramic materials that avoid the toxic lead of prior art Pb(ZrxTi1-x)O3 (“PZT”) piezoelectric ceramics and the disadvantages of prior art, lead free piezoelectric ceramics.